Telephone repeater system



Aug. 11, 1936. G. R. EATON 2,050,292

TELEPHONE REPEATER SYSTEM Filed July l5, 1953 4 Sheets-Sheet 1 RLC Aug. 11, 1936. R EATON 2,050,292

TELEPHONE REPEATER SYSTEM Filed July l5, 1933 4 Sheets-Sheet 2,

Aug. 11, 1936. Q R EATON 2,050,292

TELEPHONE REPEATER SYSTEM Filed July l5, 1933 4 Sheets-Sheet 5 Aug. 1,1, 1936. G R, EATON I 2,050,292

TELEPHONE 'REPEATER SYSTEM 4 Sheets-Sheet 4 Filed July 15, 1933 RLCA Patented Aug. 11, 1936 UNETED STATES 2,050,292 f TELEPHONE REPEATER SYSTEM George R. Eaton, Oak Park, Ill., assignor to Kellogg Switchboard and Supply Company, Chicago, Ill., a corporation of Illinois Application July 15, 1933, Serial No. 680,521

18 Claims.

This invention relates to telephone systems and more particularly to two Way repeating of telephonie currents.

An object of the invention is to produce an improved system .of two Way transmission of electrical energy over long distance lines. Such lines are of very high attenuation and amplifiers or repeaters are necessary to amplify the transmitted currents sufficiently to properly operate the receiving devices associated therewith. Due to the variable characteristics of long distance lines it has been found necessary, in order to insure proper operation of the associated repeater, to provide a network o-r artificial balancing line for association with each line. Suitable signaling and switching means are usually supplied in the link circuit used for connecting such lines.

A feature of my invention is the inclusion in the line circuit associated with the long distance lines of signaling means and switching means for said lines. y

Another feature is the association of a balancing line or network with each line or with a group of lines of similar characteristics.

A further feature is the provision of means for normally disconnecting the articial Yline or network from the long distance line when the same is idle.

Still another feature is the association with such a line circuit of a two-way repeater system of a novel and improved type.

Other features will become apparent upon a further perusal of the specification and together with the features pointed out, will be Vexplained fully hereinafter, reference being had to the accompanying drawings comprising Fig. 1 to Fig. 4 inclusive in which Fig. 1 is a circuit arrangement of a repeater line circuit `of my invention;

Fig. 2 shows the repeater cord circuit;

Fig. 3 is a circuit drawing of. the power supply circuit associated with the repeater of my invention; and

Fig. 4 is a modification of Fig. 1.

In the drawings a repeater line circuit RLC, in Fig. l, which extends by means of conductors I and 2 to a distant station (not shown) terminates in a local jack LJ at a central exchange. The line circuit RLC is provided with the usual line and disconnect signal or lamp LL, which may be any type of indicator, a repeater jack RJ, a ringing key RK, a ringing relay RR, line relay LR, cutoff relay CO, and disconnect relay DR. An artificial balancing network BN comprising coils R and RI is provided for the line circuit RLC and may be individual to a line or means may be provided for switching the artificial line in circuit with any one .of a plurality of lines having similar characteristics. Y

Fig. 2 shows a two-way repeater cord circuit 5 RCC comprising two one way vacuum tube repeaters V and VA, an answering plug AP and a calling plug CP. Ari-answering repeating coil ARC is interposed between the plug AP and vacuum tube V while a calling repeater coil CRC 10 is interposed between the calling plug CP and tube VA. Audio transformers AT and ATA are associated respectively with the output circuit of the tube repeaters VA and V. An operators repeating coil ORC is connected in the output circuit 15 of both of the tube repeaters V and VA and enables the operator to monitor or supervise the connection thru a monitors key MK. Operators relays OR and ORA are energized when the listening key LK is thrown to connect the operators 20 set tothe connected lines without the connection of the repeaters V and VA in the circuit.

The power circuit PC disclosed in Fig. 3 consists of, an alternating current transformer ACT, a rectifier tube RE, resistances 1', ra and rb, choke 25 coil cc, switching relays SW and SWA and batteries A and B.

In the modified line circuit RLCA shown in Fig. 4 the apparatus similar to that shown in Fig. 1 is marked with similar reference characters 30 with an A or a added. Additional apparatus notshown in Fig. 1 comprises an artificial line relay ALR and a key AK for initially operating the relay ALR. l

The invention may be more easily compre- 35 hended by a description of the operation of the illustrated system. Signaling current from the distant station ows over conductors land 2, closed contacts 3 and 4 of local jack LJ, normal contacts 5 and 6 of ringing relay RR, upper and 40 lower windings of relay PR and condenser c. Relay PR operates and closes alternate contact 8 during the iiow of ringing current thereby establishing an energizing circuit for the linerelay LR. This circuit can be traced from grounded 45 battery thru the Winding of relay LR, normal contact 9 of CO, alternate contact 8 of. PR to ground. The relay LR cnergizes and locks itself up thru its alternatek contact l0 and normal contact Il of relay CO. After the cessation of the 50 ow of ringing current the relay PR restores Aits contact 8 to normal and establishes a circuit for the line lamp LL or other indicator which is illuminated by current flowing from grounded battery vthru the lamp LL,"conductor I3, alternate 55.

contact l2 of relay LR, normal contact 8 of relay PR to ground.

The operator noting the display of the indicator LL, inserts the answering plug of a regular local cord circuit (not shown) into the local jack LJ to connect her to the calling party in the usual manner. Upon being informed the desired party is -located at -a distant exchange the operator withdraws 'the .plug from the local jack LJ and inserts the answering plug A P of a repeater cord circuit such as RCC into repeater jack RJ thereby operating the cut-off relay CO overa circuit extending from battery, "alternate Contact I4 of jack RJ, conductor l5, and winding of CO to ground. Relay CO energizes and interrupts the locking circuit of LR at contact lI I :and closes the talking circuit at contacts I6 and 'I1 thru to the cord circuit. Relay releases its con'- tact l2 and thereby extinguishes the line :lamp LL. A branch circuit closed at alternate contact 14 `'of RJ fpl'aces a battery 'potential upon the sleeve o'f jack LJ and `'an'y other jacks 'connected in multiple therewith to indicate 'a l'busy condition should any operator attempt to insert a plug of another 'repeater -cord therein. The closing oi alternate contact I8 completes the circuit thru the 'artificial line. If 'the operator idoes not Ahave allthe rnecessary information -to complete the cal-l s he 'next throws thelist'ening key LK which causes the operation 'o'frelafs OR and ORA overobvio'us circuits 'to ground 'atrfal'terna'te Contact '25 of R'ela'y OR closes 'its alternate contacts I`9 vand 52.6 thereby connecting the operator associated with leads 2f'l 'and 22 in -a conversational circuit with the distant station. This circuit can be traced from lead 2|, alternate 'Contact '23 -of LK, alternate :contact t9 o'f OR, 'tip 'contacts 'of `AP and RJ, resistance rc of the 'articial'line network, alternate contact I'S `of CO, normal Acontact 5 of RR, normal closed contact `3 of LJ, conductor y2 to :the 'distant station and back over conductor l, normal 'contact 4 'of normal 'contacts '6 of RR, I-l of CO, resistance To, ringV contacts of RJ and vAP,`alte1-"nate 'contacts 20 'of OR and y24 of LK, lead 22 to the 'operators set.

The operator asceta'ins the connection desired and assuming that the wanted line is 'associated with the line circuit =RLCA "of VFig. 4 inserts 'the calling rplug 'CP 'into )jack RJA which energizes relay 'COA by current ilowing from grounded battery, resistance "rea, 'al'ternate'contact l4'a, conducto'r I5a, winding of l'COA to ground. Relay COA disconnects the line relay LRA from the line at alternate contact 9a an'd closes the *balancing line lcircuit 'at contact 18a. The attendant 'next `operates 'the ringing l'key `RKA which establishes an obvious iener'g'izing circuit for -ri'nging relay RRA which closes alternate contacts 5a and V6a to 'connect "the generator leads 26a and 21a to the called line. Signaling current is sup'- plied over the leads 26 and A27 'until the key RKA is released. As -soon vas the party at the distan't en'd responds the 'operator restores the listening key LK -and `lthe `conversational 'circuit thru the repeater `system *is established.

Voice currents from vthe calling line flow over the vconductor l, normalcontactsand 6 and the lower heavily marked conductors to jacks RJ rand A-P, normal contact 20 of OR, :secondary winding s vof ythe transformer 'AT, condenser C, midpoint of the fleft hand winding tof repeating -coil ARC, upper winding u of vARC and the upper heavily marked `conductors to conductor Y2 and over "the line. Current passing -thru 'coil u of ARC Acauses afilowofinduced current thru the right hand winding rh of ARC which is impressed upon the grid g of the repeater V and transmitted to the plate p and over the heavily marked conductor to primary winding pa of ATA, lower right hand Winding Zrh of operators repeating coil ORC, conductor 62 to a source of current in Fig. 4. Current is induced in the secondary sa of ATA which current flows thru condenser CI to the mid-point of the right hand winding of CRC and thence in parallel, one path extending thru the upper winding ua of CRC, normal contact 3| of ORA, tip contacts of CP and RJA, resistance rca, alternate contact EG-a of COA, normal contact 5a of RRA, normal contact 3a of LJA, `out over the line and back thru normal contacts 4a and 6a, alternate contact lla of COA, resistance rda, ring contacts of RJA and CP, normal contact 32 of ORA to sa; the other path extending thru the bottom winding ba of CRC, normal contact 33 of ORA, sleeve contacts of `Cl? and RJA, conductor` 3G, alternate contact ifea fof COA, condenser 35a, left hand winding 36a of resistance RA to the inid-point of RA where it Idivides and part `ilows thru resistance 32a, normal contact A38 of ALR, to the mid-point of resistance RI-A and part flows thru resistance Ma, normal 'contact 42a 4of RRA, conductor 43a, resistances Ma, 55a, and Ma, to the midpoint of R-IA 'where it joins the rst path and both il'ow thru resistance 39a, conductor Qa to the heavily marked conductor and back to sa. The parallel pa'ths pass respectively thru the physical line to 'the dista-nt station and 'thru the balancing network BNA 'which is so constructed as to have the same characteristics as has the physical line. The ow of current thru upper winding z'ta a'ii'd bottom winding ba Vis equal in e'ach case and dueto the construction of CRC neiitr'alize `each other and prevent any cur-rent ben'g'in'd'ced inthe -left hand winding 47 of CRC and setting up voice currents in the repeater VA.

Voice currents originating at the called line p'ass tliru'th'e line circuit VRLCA over the heavily marked conductorsithiu the tip and ring contacts of 'CP, normal 'contacts 3| and 32 of ORA, winding sa oi ATA, vcondenser :Cl and upper 'winding ua o'f CRC. Current -iio'wing thru ua induces a flow of current thru winding 47 which is impressed'upon the grid g'of repeater VA and thence flows iromplate p of VA, thru the primary winding p of AT and upper 'right hand winding urli of 'ORC and 'conductor 62 to a source of plate current.

Induced current flows thru the secondary s of AT, condenser 'C to the mid-point of the left han'd Winding of where it divides 'equally and ilows over parallel paths having identical characteristics. One-half flows thru upper Winding u of ARC, the heavily 4marked conductor thru the tip contacts of VAP and RJ, resistance rc, alternate'contact l5 'of CO, normal contacts 5 and 3, over the lon'g line to the 'distant station and back on conductor l, normal contacts 4 and 5,'alte1- nate contact l l, resistance rd, ring contacts of RJ and 'normal contact '2B of OR, to the secondary -s of AT. The other `half of the current flows thru the lower winding b of ARC, normal contact 50 of OR, sleeve contacts of AP and RJ, conductor 34, alternate contact Hl of CO, conductor 35, 'resistance 35, where it divides and one part flows 'thru 'resistance 3l to the mid-point of Rl Iand the 'other part flows thru resistance 4l, normal contact 42 of RR, `resistances fifi, 55, and 3S Ito the mid-pointof Rl where the paths meet a-nd pass thru resistance 39, 'conductor 40, where it joins the rst mentioned path and returns to the secondary s of AT. The current flowing in the windings u and b of ARC neutralize each other and consequently no flow of current is set up in the secondary rh of ARC. l.

Should the operator desire to listen in on the connection for monitoring purposes she operates the monitoring key MK which connects the operators set by its contacts 5I and 52, over conductors 2| and 22, to the secondary windings os and osa whereinduced current from the conversational circuit is flowing. If the connection is not operating properly the operator may operate the key AK of RLCA which operates the relay ALR by current flowing from battery thru resistance rea, operated key AK, the winding of ALR, conductor 53, normal contact 54 of DRA. Relay ALR interrupts the circuit between coils rca and rela at contact 55, and opens the circuit of the balancing network BNA at alternate contact 38 thereby permitting a gain over the physical line with improved clarity of reception of the transmitted message. The closure of alternate contact 51 of ALR establishes a branch circuit for the relay COA which may be traced from battery, resistance rea, alternate contact 51 of ALR, conductor I5a, thru the winding of CO-A to ground.

YUpon the cessation of the conversation, signaling current may be impressed upon the line circuits at the distant stations causing the operation of the disconnect signals shown as lamps LL'and LLA in the following manner. The signaling current operates PR as before explained which closes alternate contact B to energize the disconnect relay DR by current flowing from grounded battery thru the winding of DR, alternate contact 9 of CO, to ground at 8 of PR. Disconnect relay DR closes a locking circuit for itself thru alternate contacts 58 and Il and an operating circuit for disconnect lamp LL or other signal thru alternate contact 56. The operator noticing the lighted lamp LL withdraws the plug AP from RJ which interrupts the energizing circuit of cut oli relay CO which restores contact Il to normal to deenergize DR which extinguishes disconnect lamp LL and thereby restores all the apparatus to normal. The lamp LLA of RLCA islighted in a similar manner and the plug CP is withdrawn from the jack RJA. However, in this case the cut-off relay COA is not deenergized as the branch circuit thru alternate contact 51 of ALR is still closed. The restoration of AK interrupts the energizing circuit of ALR which restores relay COA to normal by opening contact 51 with the subsequent restoration of disconnect-relay DRA and the extinguishment of the disconnect lamp LLA which places the apparatus in condition for another call. l

Current for the repeater tubes V and VA is supplied by the power circuit PC of Fig. 3 thru conductors-GU, 6I, 62, and 63. The current used is normally supplied by a-source of alternating current such as is ordinarily used Vfor lighting purposes over conductors 64 and 65 or (i4-and 66, depending upon the voltage of the line, to the primary winding 61 of a transformer ACT. Condensers 68 and 69 having their mid-point grounded are connected across the conductors '64 and 55 to filter outlany high frequency'current which may be present on the ACv line. The secondary of ACT is divided into three portions HW, RHWand PW; the coil HW supplies heating current to the filaments f and f of the, repeaters V and VA over conductors 60 and 6l; theA rectifier heater winding RHW supplies heatingcurrent to the filament fa voi'the rectiertube RE and the winding PW supplies current tothe plate elements p of the rectifier RE.

The plate current for V andVA hasone pole connected to the mid-'point of RHW which is the 5 positive pole, and vthen vthru a choke coil cc, resistance rb to provide the proper potential, alternate contacts 69 of switching relay SWA which is normally operated, over conductor 52 to the plates p and p of V and VA.l The negative sidel of 10 the plate current is from the mid-point of PW, conductor 63 to the grids g of V and VA, and thru V and VA to` the iplates p. The grids g are kept negativeto the cathodes or filaments f by a resistance r which isV connected between groundv and the mid-point of winding HW. A condenser C10 is connected across the resistance r, and condensers c1| and 012 are for filtering out any undesirable hum or other noises. A portion of the plate current passes thru the relays SWAand SW 20 and maintains them operated as long as plate current is passing thru conductor 62. Relay SWA closes alternate contact 691:0 complete the plate current circuit and opens contact 13. Relay SW closesalternate` contacts 14 and 15 which connect 25 the ends of HW to conductors 60 and Si to supply heating current to j and f. If the rectiiier RE or the sourceof alternating current fails the relays SW and SWA deenergize and restore their contacts to normal. The restorationrof contacts 14 and 15 connect the conductors'BU and 6I to the positive and negative terminals of an emergency battery A which supplies heating current to the filaments f. Contact G9 disconnects positive plate current conductor 62 from the winding,v RHW and connects it to emergency battery B which thereafter supplies plate current to p and p of V and VA. vThe closure of contact 13 of SWA maintains the negative bias on the grids g by` connecting the negative pole of battery A to ground thru resistance 1'. Upon the resumption of a flow of current thru the rectier RE the relays SW and SWA operate to disconnect the remergency batteries A and Bkfrom the supply conductors 60, 6|and 62.

yWhile I have described my invention in certain particular embodiments, I desire that it be understood that modifications and alterations rray be made and that I intend no limitations upon my invention otherthan those imposed by the scope of the appended claims.

Having described kmy invention, what I claim as new and desire to `secure by United States Letters Patent is:

1. In a telephone system a transmission line, a jack in which said line terminates, a primary relay associated with said line and arranged to be. operated by signaling current thereover, a line relay, a cut-ofgrelay, an energizing circuit for said cut-oli relay, a contact associated with va 60 jack controlling saidcircuit for said cut-off relay,

a circuit for said linev relay controlled by said primary relay and said cut-off relay, an indicator for said line, anda circuit for said indicator including contacts of saidline relay and said primary relay.

2. In a telephone system, a transmission line, a jack for said line', a primaryl relay associated with said line and arrangedto ybe actuated by signaling current thereover, a line relay for said f contact of said primary relay and a back contact 75- of said c11`t-'oil"'relay, an indicator for said line, 'and a circuit for said indicator including a Vfront contact of said line relay and a back lContact of said primary relay.

3. In a telephone system, a transmission line, a jack in which said line terminates, 'a primary relay associated With said line and arranged to be 'actuated by signaling current thereover, 'a disconnect relay vfor said line, a cut-off relay, an energizing vcircuit for said cut-off relay, a contact associated with a, jack controlling said circuit for said cut-01T rela'y, a. circuit for said disconnect relay 'i'ncluding a contact of Vsaid cut-off relay 'and va contact of said primary relay, an indicator for said line, and a circuit for Said indicator including a contact of said disconnect relay and 'a contact of said primary relay.

4. In a telephone system, a transmission line, ajack `in which said line terminates, a primary relay associated with said line and arranged to be 'actuated by signaling current vthereover, a line relay for said line, a cut-off relay, a circuit for said line relay extending thru a contact lof said primary relay and la contact of said cut-off relay, an indicator for said line, acircuit for saidindicator including'a contact of said line relay and a contact of said primary relay, a circuit for said cut-olf relay, and means actuated incident to inserting Va plug in said jack for completing 'said cut-oli.D relay circuit.

5. In a Vtelephone system, a transmission line, a jack in which said lines terminates, a primary relay associated with said line and arranged to be actuated by signaling current thereover, a line relay for said line, a cut-off relay, a circuit for said line relay extending thru a front contact of said primary relay and a back contact of said cut-off relay, a locking circuit for said line relay including a back contact of said cut-01T relay, an indicator for said line, a circuit for said indicator including a front contact Yof said line relay and a back contact of said primary relay, a circuit for said cut-oir relay, and means actuated incident to inserting a plug in said jack for closing said cut-oir relay circuit.

6. In a telephone system, -atransmission line, a jack in which said line terminates, a primary relay associated With said line and arranged to be actuated by signaling current thereover, a disconnect relay for said line, a cut-oilD relay for said line, a circuit for said disconnect relay including a contact of said primary relay and a contact 'of said cut-off relay, an indicator for said line, a circuit for said indicator including afcont'act o'f said disconnect relay and a contact of Isaid primary relay, 'a circuit for said cut-off relay, and means for completing said circuit act'uated incident to inserting a plug in said jack.

'7. In a telephone vsystem a transmission line, a jack Vin Which said line terminates, a primary relay associated with saidV line arranged to be actuated 'by signaling current thereover, a disconnect relay, a cut-off relay, a circuit for said disconnect relay including a front contact of said cut-olf relay and a front contact of said primary relay, an indicator for said line, a circuit for said indicator including a front contact of said disconnect relay and va back contact of said primary relay, -a circuit for said cut-oi relay, and means -for completing said cut-off relay circuit incident to inserting 'a plug in said jack.

8. "In a telephone system, 'a transmission line, a jack in which said line terminates, a primary relay associated with -said line and arranged to be *actuated by signaling current thereover, a line relay for said line, a cut-off relay, a circuit for said relay including a back contact of said cutoiT relay and a front contact of said primary relay, an indicator for said line, a circuit for said indicator including a Yfront contact of said line F5 relay and a back contact of said primary relay, a vdisconnect relay, a circuit for said disconnect relay including a front contact of said cut-olf relay and a front contact of said primary relay, a circuit for said signaling device including a l0 front contact of said disconnect relay and back contact of said line relay and a back contact of said primary relay, a circuit for said cut-off relay, and means for completing said circuit incident to inserting `a plug in said jack. 1'5

9. In a telephone system, a transmission line, a jack in which said line terminates, a ringing relay for said line, a circuit for said ringing 'relay, a key for completing said circuit, a ringing generator, leads from said generator terminatl0 ing in front contacts of said relay, and back contacts for vsaid relay for cutting oir said jack from said transmission line upon actuation of said key.

l0. In a telephone system a transmission line, 2 5 a jack in which said line terminates, a primary relay associated with said line and arranged to be operated by signaling current thereover, a line relay, a cut-off relay, a circuit for said line relay controlled by said primary relay and said cut-oli relay, an indicator for said line, a circuit for said indicator including contacts of said line relay and said primary relay, and a locking circuit vfor said line relay closed at its own front contact and a back contact of said cut-ori" relay.

1l. In a telephone system, a transmission line, a jack for said line, a primary 'relay associated With said line and arranged to be actuated by signaling current thereover, a line relay for said line, a cut-o relay, a circuit for said line -relay including a front contact of said primary relay and a back contact of said cut-oir relay, an indicator for said line, a circuit for said indicator including a front contact of said line relay and a back contact of said primary relay, and a 215 locking circuit for said line relay controlled by said cut-oir relay.

l2. In a telephone system, a transmission line, a jack in which said line terminates, a primary relay associated With said line and arranged to be actuated by signaling current thereover, a disconnect relay for said line, a cut-oli" relay, a circuit for said disconnect relay including a contact of said cut-off relay and a contact of said primary relay, an indicator for said line, a circuit for said indicator including a contact of said disconnect relay and a contact of said primary relay, and a locking circuit for said disconnect relay controlled by said cut-off relay.

13. In a telephone system, a transmission line, a jack in which Ysaid line terminates, a primary relay associated with said line and arranged to be actuated by signaling current thereover, a line relay for said line, a cut-off relay, a circuit for said line relay extending thru a Contact of said primary relay and a contact of said cut-oir' relay, an indicator for said line, a circuit for said indicator including a contact of said line relay and a contact of said primary relay, a circuit for said cut-01T relay, means actuated incident to inserting a plug in said jack for completing said cutoff relay circuit, and a locking circuit for said line relay controlled by said cut-off relay.

14. In a telephone system, a transmission line, a jack in which said line terminates, a primary 75 relay associated with said line and arranged to be actuated by signaling current thereover, a disconnect relay for said line, a cut-oi relay for said line, a circuit for said disconnect relay including a contact of said primary relay and a contact of said cut-off relay, an indicator for said line, a circuit for said indicator including a contact of said disconnect relay and a contact of said primary relay, a circuit for said cut-oil' relay, means for completing said circuit actuated incident to inserting a plug in said jack, and a locking circuit for said disconnect relay controlled by said cut-off relay.

15. In a telephone system a transmission line, a jack in which said line terminates, a primary relay associated with said line arranged to be actuated by signaling current thereover, a disconnect relay, a cut-off relay, a circuit for said disconnect relay including a front contact of said cut-off relay anda front contact of said primary relay, an indicator for said line, a circuit for said indicator including a front contact of said disconnect relay and a back contact o said primary relay, a circuit for said cut-oil relay, means for completing said cut-off relay circuit incident to inserting a plug in said jack, and a locking circuit for said disconnect relay controlled by cut-off relay.

16. A telephone system comprising a plurality of lines, a link circuit for interconnecting said lines, a voice current repeater in said link circuit, a balancing network associated with one of said lines having characteristics similar to those of the said line, a three-conductor contact switch for connecting said link circuit to said line, a talking circuit extending thru two of the contacts of said switch, and a circuit joining said balancing network to said link circuit and repeater including two contacts only of said switch.

17. A telephone system comprising a, plurality of lines, a link circuit for interconnecting said lines, a voice current repeater in said link circuit, a balancing network associated with one of said lines having characteristics similar to those of the said line, a three-conductor switch for connecting said link circuit to said line, a two-wire talking circuit extending thru two of the contacts of said switch, and a circuit joining said balancing network to said line and link circuit and repeater includingone of said pair of talking contacts and the third contact of said switch.

18. A telephone system comprising a plurality of lines, a cord circuit for interconnecting said lines, a voice current repeater in said cord circuit, a balancing network associated with one of said lines having characteristics similar to those of the said line, a three-conductor plug and jack for connecting said cord circuit to said line, a two-Wire talking circuit extending thru two of the contacts of said plug and jack in series and a circuit joining said balancing network to said line and cord circuit repeater including one only of the talking contacts of said pair and the third contact of said plug and jack.

GEORGE R. EATON. 

